This invention relates to a tundish impact pad, i.e. a pad of erosion resistant material positioned on the floor of a tundish to receive the incoming stream of molten metal poured into the tundish from a ladle.
There have been various recent proposals to improve the design of impact pads, particularly with a view to eliminating, or at least reducing, surface turbulence in a continuous casting tundish, to minimise slag entrainment within the liquid steel bath in the tundish, to prevent break-up of tundish flux cover and reoxidation of the liquid steel bath and to ensure a proper flow path. These designs, essentially, have included a base for the pad and upstanding walls around the perimeter of the base to define an opening into which the molten metal stream is received.
Thus, for example, in U.S. Pat. No. 5,169,591 is disclosed an impact pad having a base to receive an incoming ladle stream and one or more sidewalls extending upwardly along the periphery of the base. Each sidewall has an inner surface having an undercut portion facing the incoming ladle stream and extending along the length of the inner surface, thereby providing a shaped surface to receive and reverse the direction of fluid flow generated by the incoming ladle stream.
U.S. Pat. No. 5,358,551 discloses an impact pad having a bottom impact surface and an endless sidewall extending upwardly from the bottom impact surface and defining an opening into which molten metal is poured. The inner surface of the endless sidewall includes an annular portion which extends inwardly and upwardly towards the opening. Preferably the inner surface of the sidewall curves continuously from the bottom impact surface to a vertical wall defining the opening.
It is an object of the present invention to provide an improved impact pad.
It is a further object to provide an impact pad which results in improved flow characteristics of molten steel poured into a tundish, particularly for a longitudinally-extending tundish in which the steel input zone and steel outlet zone(s) are towards opposite ends of the tundish.
Accordingly, in one aspect the invention provides a tundish impact pad comprising a body of refractory material capable of withstanding contact with molten steel, the body comprising a base having an impact surface for molten steel, an outer sidewall extending upwardly from the impact surface, the outer sidewall extending around the base to completely enclose it, an annular body portion connected to the sidewall and providing a top surface substantially parallel to the impact surface and defining an opening into which molten steel can be poured, the lower face of the annular body portion and the inner face of the sidewall defining a recess having an undercut portion extending continuously around and above the impact surface, characterised in that a portion of the top surface is at a lower level than the remainder of the top surface and the recess beneath the portion of the top surface is of smaller cross-section than the remainder of the recess.
Preferably the lower face of the annular body portion and the inner face of the sidewall provide a continuous curved surface which, at its lower end, merges into the impact surface of the base thereby providing a concave recess around the base, the recess having two discontinuities or steps, i.e. one at each end of the smaller recess portion.
The impact pad is preferably four-sided in plan view and, in a particularly preferred embodiment, it has one pair of parallel sides, (defined by the outer surface of the sidewall) i.e. it is wedge-shaped. In this latter embodiment, the longer parallel side preferably includes the first portion of the top surface, i.e. includes the lower top surface portion, and the smaller recess. Alternatively, the pad may be, for example, rectangular in plan view.
The smaller recess is preferably not coterminous with the lower top surface portion and it is especially preferred that the smaller recess portion does not extend as far as the ends of the lower top surface portion, i.e. the smaller recess portion is not as long as the lower top surface. Thus, in the preferred wedge-shaped pad construction, the lower top surface portion stops a little short of each end of that sidewall to provide a step up to a top surface level with the remainder of the surface extending around the other sides of the pad and the smaller recess portion extends underneath the lower top surface portion sufficiently that each of its ends overlaps the recess in its respective adjacent side of the pad but stops short of the aforesaid step.
In this embodiment, therefore, there is in effect a cut-out portion in the top surface along one of the four sides of the pad.
As indicated above, the impact pad of the invention is particularly intended for use with a longitudinally-extending tundish having a steel inlet zone towards the end opposite to the outlet zone(s).
Accordingly, in another aspect the invention provides a tundish having an inlet zone adjacent a first end thereof and an outlet zone adjacent the opposite end thereof, an impact pad positioned on the floor of the tundish in the inlet zone, the impact pad comprising a body of refractory material capable of withstanding contact with molten steel, the body comprising a base having an impact surface for molten steel, an outer sidewall extending upwardly from the impact surface, the outer sidewall extending around the base to completely enclose it, an annular body portion connected to the sidewall and providing a top surface substantially parallel to the impact surface and defining an opening into which molten steel can be poured, the lower face of the annular body portion and the inner face of the sidewall defining a recess having an undercut portion extending continuously around and above the impact surface, characterised in that a portion of the top surface is at a lower level than the remainder of the top surface and the recess beneath the portion of the top surface is of smaller cross-section than the remainder of the recess and the pad is positioned with the first portion of the top surface facing to the first end of the tundish.
In the particularly preferred embodiment, therefore, where the impact pad is of wedge-shape, the longer parallel side of the pad is positioned facing the first end of the tundish.
The impact pads of the invention have been found to be particularly useful in reducing surface turbulence in tundishes of the type described above and are equally useful in tundishes which are generally rectangular in plan view and tundishes which are generally of wedge shape, i.e. with two parallel sides, in plan view. Moreover, the flow patterns of the molten metal within the tundish can be improved over those resulting from use of conventional impact pads.
The impact pads may be formed from a castable refractory composition capable of withstanding continuous contact with molten metal, in particular molten steel such as is used in continuous casting operations. Usually a standard medium-to-high alumina refractory with an alumina content in the range of about 55% to 85% by weight is desirable. Where a basic refractory is preferred because of steel chemistry, it is preferred that a magnesia-based refractory composition be utilised, with MgO in the range of about 58% to 93% by weight.
The invention is now described by way of example only with reference to the accompanying drawings in which: